The importance of achieving accurate mutual alignment of individual components in any optical system is well known. The miniature dimensions of components used in modern optical communication systems render such alignment difficult both to achieve and to maintain. For example, one problem in the construction of laser transmitters is that of efficiently coupling the optical output from a laser diode into an optical fiber. To obtain efficient coupling, the fiber end is desirably precisely aligned with the emitting area of the laser. When such alignment is achieved, the fiber is then fixed in place, ideally by a method that ensures alignment is sustained throughout the device lifetime.
Typically, fiber-coupled diode lasers are packaged in gold plated metal packages and the fiber is held in alignment with the laser using either epoxy, laser weld, or solder attachment techniques with or without a ferrule. Epoxy attachment is low cost but may have too much thermal expansion for high precision attachments. Furthermore, it is not reliable over a long period of time due to outgassing and alignment shifts arising from aging and temperature cycling. Laser weld techniques are reliable but require costly ferrulization of the fiber and specially designed mounts or clips to allow weld attachment of the ferrulized fiber. The mounts/clips are expensive, large, and may creep over time. Solder attachment techniques, on the other hand, are reliable and low cost, and have become prevalent in the art. However, existing solder attachment techniques tend to use either an integrated heating mechanism and/or a specially configured platform to isolate the heat for a solder reflow. These tend to be expensive and undesirably large.
The mounting point at which the fiber is soldered desirably has specific material properties in order to work effectively. An acceptable material for the mounting point desirably has a low thermal conductivity (e.g. less than 50 W/m-K) and a thermal expansion coefficient that maintains fiber alignment while the package is heated. The exact thermal expansion property desired may depend on the material to which the laser is mounted, the respective thickness of the fiber mount and laser submount, and/or the temperature profiles expected during operation. The fiber mount material also may be able to be soldered or be able to be plated with a solderable material. During the soldering process, the fiber mount may experience significant stress resulting from differential expansion due to temperature gradients and materials differences. Therefore, the fiber mount desirably has a high tensile strength (e.g. greater than 25 kpsi) to avoid fracturing.
It is difficult, however, to maintain alignment between the optical component and the fiber when the fiber soldered due to turbulent flows and capillary forces exhibited by the molten solder.